1. Field of the Invention
The present invention relates to cryogenic cooling apparatus. More specifically, the present invention relates systems and techniques for reducing-thermal noise in cryostats.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
In a traditional Joule-Thompson cryostat, a high pressure gas such as nitrogen is pre-cooled and converted to a cryogenically cool liquid on expansion in a cooling volume. The liquid is used to cool a cold finger, which in turn can be used to cool, for example, an infrared (IR) sensor. The liquid boils into a gas and is sent through heat exchanger fins to cool the incoming high-pressure warm gas.
Temperature at the cold finger of a cryostat is found to vary significantly, resulting in "thermal noise". Any variation in temperature causes changes in the output signals of the DC-coupled IR sensors. Because the changes vary for each IR sensor, short-term spatial noise is induced on the output scene, with a corresponding decrease in sensitivity. The major sources of thermal noise are effects that change the pressure in the area where the liquid nitrogen is boiling, since the temperature of the boiling gas is a strong function of the absolute pressure. Flow resistance in the fins, necessary for efficient pre-cooling, converts modulation of the gas flow rate into pressure modulation, resulting in thermal noise. Liquid/gas phase changes alter the mass flow rate of the nitrogen. A change in the mass flow rate results in changes in the pressure in the cooling volume, the temperature of the liquid coolant, the ratio of liquid to gas, and the temperature and flow rate of gas flowing back from the cooling volume to the pre-cooler. The overall affect is that the cryostat flow rate oscillates due to negative thermal feedback. Because the output of the high-pressure pre-cooler line is returned to pre-cool the incoming gas and the mass flow rates are temperature sensitive, temperature oscillation occurs, producing thermal noise.
Prior efforts have focused on filtering out thermal noise, rather than reducing its causes. One filtering method involves increasing the thermal mass in order to increase the thermal time constant. Increasing the thermal mass has the disadvantage of increasing cool-down times, which can be unacceptable for tactical systems. Another noise-reduction approach is to use longer electronic filter time constants (integration time) on the electronic output of, for example, IR detectors. The disadvantage of longer electronic time constants is that they require a detector to dwell on a given scene to maintain sensitivity or increase cost by requiring more detectors to achieve the same scan times.
Thus, there is a need in the an for a short-term, thermally stable cooling cryostat with reduced temperature variation due to flow rate modulation.